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<b>请注意:</b><br>本书基于 Elasticsearch 2.x 版本，有些内容可能已经过时。
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<span class="breadcrumb-node">文档元数据</span>
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<h2 class="title">
<a id="_Document_Metadata"></a>文档元数据<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/030_Data/05_Document.asciidoc">edit</a>
</h2>
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<p>一个文档不仅仅包含它的数据 ，也包含 <em>元数据</em> —— <em>有关</em> 文档的信息。
三个必须的元数据元素如下：</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
<code class="literal">_index</code>
</span>
</dt>
<dd>
文档在哪存放
</dd>
<dt>
<span class="term">
<code class="literal">_type</code>
</span>
</dt>
<dd>
文档表示的对象类别
</dd>
<dt>
<span class="term">
<code class="literal">_id</code>
</span>
</dt>
<dd>
文档唯一标识
</dd>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_index"></a>_index<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/030_Data/05_Document.asciidoc">edit</a>
</h3>
</div></div></div>
<p>一个 <em>索引</em> 应该是因共同的特性被分组到一起的文档集合。
例如，你可能存储所有的产品在索引 <code class="literal">products</code> 中，而存储所有销售的交易到索引 <code class="literal">sales</code> 中。
虽然也允许存储不相关的数据到一个索引中，但这通常看作是一个反模式的做法。</p>
<div class="tip admon">
<div class="icon"></div>
<div class="admon_content">
<p>实际上，在 Elasticsearch 中，我们的数据是被存储和索引在 <em>分片</em> 中，而一个索引仅仅是逻辑上的命名空间，
这个命名空间由一个或者多个分片组合在一起。
然而，这是一个内部细节，我们的应用程序根本不应该关心分片，对于应用程序而言，只需知道文档位于一个 <em>索引</em> 内。
Elasticsearch 会处理所有的细节。</p>
</div>
</div>
<p>我们将在 <a class="xref" href="index-management.html" title="索引管理"><em>索引管理</em></a> 介绍如何自行创建和管理索引，但现在我们将让 Elasticsearch 帮我们创建索引。
所有需要我们做的就是选择一个索引名，这个名字必须小写，不能以下划线开头，不能包含逗号。我们用 <code class="literal">website</code> 作为索引名举例。</p>
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<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_type"></a>_type<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/030_Data/05_Document.asciidoc">edit</a>
</h3>
</div></div></div>
<p>数据可能在索引中只是松散的组合在一起，但是通常明确定义一些数据中的子分区是很有用的。
例如，所有的产品都放在一个索引中，但是你有许多不同的产品类别，比如 "electronics" 、 "kitchen" 和 "lawn-care"。</p>
<p>这些文档共享一种相同的（或非常相似）的模式：他们有一个标题、描述、产品代码和价格。他们只是正好属于“产品”下的一些子类。</p>
<p>Elasticsearch 公开了一个称为 <em>types</em> （类型）的特性，它允许您在索引中对数据进行逻辑分区。不同 types 的文档可能有不同的字段，但最好能够非常相似。
我们将在 <a class="xref" href="mapping.html" title="类型和映射">类型和映射</a> 中更多的讨论关于 types 的一些应用和限制。</p>
<p>一个  <code class="literal">_type</code> 命名可以是大写或者小写，但是不能以下划线或者句号开头，不应该包含逗号，
并且长度限制为256个字符. 我们使用 <code class="literal">blog</code> 作为类型名举例。</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_id"></a>_id<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/030_Data/05_Document.asciidoc">edit</a>
</h3>
</div></div></div>
<p><em>ID</em> 是一个字符串，当它和 <code class="literal">_index</code> 以及 <code class="literal">_type</code> 组合就可以唯一确定 Elasticsearch 中的一个文档。
当你创建一个新的文档，要么提供自己的 <code class="literal">_id</code> ，要么让 Elasticsearch 帮你生成。</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_Other_Metadata"></a>其他元数据<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/030_Data/05_Document.asciidoc">edit</a>
</h3>
</div></div></div>
<p>还有一些其他的元数据元素，他们在 <a class="xref" href="mapping.html" title="类型和映射">类型和映射</a> 进行了介绍。通过前面已经列出的元数据元素，
我们已经能存储文档到 Elasticsearch 中并通过 ID 检索它—​换句话说，使用 Elasticsearch 作为文档的存储介质。</p>
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